public void Execute()
{
// using the original superclass, the client can access the old functionality
IDoesSomething oldFunctionality = new LegacyClass();
oldFunctionality.DoSomeThing("foo", 5);
// new functionality A can be accessed via the FunctionalityA class,
// which additionally provides the old functionality
IDoesSomething functionalityA = new FunctionalityA();
oldFunctionality.DoSomeThing("foo", 5);
// new functionality B can be accessed via the FunctionalityB class,
// which also provides the old functionality
IDoesSomething functionalityB = new FunctionalityB();
oldFunctionality.DoSomeThing("foo", 5);
// what happens when we want both A and B?
// IDoesSomething functionalityAandB = new FunctionalityAAndB(); ?
// what when we want A and C but not B?
// when functionalities C-Z or more are added, we need either an unwieldy
// subclass hierarchy or some convoluted logic.
}
public void Execute()
{
// using the original class, the client can access the old functionality
IDoesSomething oldFunctionality = new LegacyClass();
oldFunctionality.DoSomeThing("foo", 5);
// now adding functionality A; each decorator is constructed with the interface that it implements
IDoesSomething functionalityWithA = new LegacyClassDecoratorA(new LegacyClass());
functionalityWithA.DoSomeThing("foo", 5);
// similar pattern for functionality B on top of the old functionality..
IDoesSomething functionalityWithB = new LegacyClassDecoratorB(new LegacyClass());
functionalityWithB.DoSomeThing("foo", 5);
// combinations of functionality are possible with constructor chaining - original + A + C
IDoesSomething functionalityWithAandC = new LegacyClassDecoratorC(new LegacyClassDecoratorA(new LegacyClass()), 10);
functionalityWithAandC.DoSomeThing("foo", 5);
// the .Net streams library is a good example of the decorator pattern in the wild:
using (var outputStream =
new GZipStream(
new BufferedStream(
new FileStream("hello.txt.gz", FileMode.Create)), CompressionMode.Compress))
{
byte[] someData = Encoding.UTF8.GetBytes("Hello world!");
outputStream.Write(someData, 0, someData.Length);
outputStream.Flush();
}
}